Revealing mistake: When Gail throws a stone to make it bounce on the water you see in slow motion that she fakes the throw with an empty hand, and only in the next shot do you actually see the stone bouncing. (00:27:25)
Revealing mistake: In the final showdown, when Gail points the gun at Wade who is standing in the water, the raft isn't moving altough the current is pretty fast and the raft didn't get stuck anywhere as the cable is up in the air again. (01:37:55)
Revealing mistake: In the scene where Tom and the dog Maggie are having to jump off the cliff, Tom jumps first and then tries to persuade "Maggie" to jump after him. She finally does, but as she leaps, the camera is below her, revealing male dog anatomy.
Chosen answer: He's finally had enough of Wade's increasingly psychotic behavior and no longer trusts him. As a result of Wade's actions, their entire plan is unraveling. Terry likely knows that Wade will probably kill him as well.
raywest